1. Field of the Invention
This invention relates to a dampening water feeding method for a printing machine that controls an ink feeding rate and a dampening water feeding rate by measuring the color density of prints produced, and to the printing machine to which this method is applicable.
2. Description of the Related Art
Such a printing machine includes ink feeders for adjusting the rates of feeding inks to ink rollers. Each ink feeder has a plurality of ink keys juxtaposed in a direction perpendicular to a direction for transporting printing paper in time of printing. The rate of feeding ink to the ink rollers is adjusted by varying the opening degree of each ink key. In this way, the rate of feeding ink ultimately to the printing plate is adjusted.
The printing machine further includes dampening water feeders for adjusting the rate of feeding dampening water to water rollers. Each dampening water feeder includes a water source having a water vessel for storing dampening water and a fountain roller rotatable by a motor, and water rollers for transferring dampening water from the fountain roller to the surface of the printing plate. Such a dampening water feeder can adjust the rate of feeding dampening water to the surface of the printing plate by varying the rotational frequency of the fountain roller.
The printing plate has regions called detecting patches or control strips formed in positions corresponding to the respective ink keys. The opening degree of each ink key and the rotational frequency of the water fountain roller are adjusted by measuring, with a densitometer, the color density of the corresponding detecting patch actually printed on printing paper.
During a printing operation of the printing machine, a phenomenon called ink slagging may occur due to some cause. This ink slagging is a phenomenon in which background scumming (ink tinting) appears on blank portions (unprinted areas) of printing paper due to a relative shortage of dampening water.
Heretofore, generally, a determination of such ink slagging relies on a visual confirmation by the operator observing the printing paper printed. For this reason, inappropriate prints due to ink slagging could be produced in large quantities.
The object of this invention, therefor, is to provide a dampening water feeding method for a printing machine, and a printing machine, capable of avoiding inappropriate prints by detecting ink tinting and adjusting a dampening water feeding rate.
The above object is fulfilled, according to this invention, by a dampening water feeding method for a printing machine that controls an ink feeding rate and a dampening water feeding rate by measuring, with a color density measuring device, color density of prints produced, the method comprising a color density measuring step for measuring color densities of unprinted areas on prints with the color density measuring device, a determining step for determining from the color densities obtained in the color density measuring step whether ink tinting has occurred, and a dampening water adjusting step for adjusting the dampening water feeding rate when the determining step determines that ink tinting has occurred.
The above dampening water feeding method for a printing machine, and the printing machine, adjust the dampening water feeding rate upon detection of ink tinting. Consequently, inappropriate prints noted hereinbefore may be avoided.
In a preferred embodiment, the dampening water adjusting step is executed to adjust the dampening water feeding rate after feeding dampening water in an increased rate once.
Preferably, a variation in the ink feeding rate is prohibited for a predetermined waiting time after feeding dampening water in the increased rate once in the dampening water adjusting step.
In another preferred embodiment, the color density measuring device includes an image pickup unit for picking up images of printed printing paper transported toward a discharge unit, the color density measuring step being executed to detect the color densities of the unprinted areas on the prints with the image pickup unit.
In a further aspect of this invention, a printing machine comprises a color density measuring device for measuring color densities of unprinted areas on prints, and a control device for determining from the color densities measured by the color density measuring device whether ink tinting has occurred, and adjusting a dampening water feeding rate when ink tinting is determined to have occurred.
Other features and advantages of the invention will be apparent from the following detailed description of the embodiments of the invention.